How to switch different sequences of characters with Animation Nodes?
Oh that's an interesting problem. Here is my solution:

I started by making a group that 'interpolates' between two characters (numbers in this case, but I wanted it to be more generic).
The group takes the Start and End character as well as a list of the ordered characters. Additionally it has a Step input. The output is the current interpolated character after the given amount of steps.



With that given the first effect you described is fairly easy to archieve:



The second one is a little more complicated because we have to calculate the amount of required steps to get the timing right. The group that does that is very similiar to the other group. 





The two most important nodes I used to archieve the effects are the Search List Element and Shift List node. This screenshot hopefully shows how they work:



[EDIT:]
To have a text list as input you can reuse most of the existing nodes. The final setup looks like so (I don't show the nodes that did not change here):